Recently I had to investigate whether Cisco Router and Security Device Manager (SDM) uses a default username/password, so I was forced to figure out how exactly the SDM works. The minimum functionality needed on an SDM-enabled router is preconfigured Ethernet interface, HTTP server and (optionally) DHCP pool. Just in case you need to reset a router configuration into a state where you can reinitialize it with SDM, here's the minimum configuration to make it work:
! Use whatever your first Ethernet interface is hereNote: routers shipped from Cisco would usually include username cisco password cisco configuration command.
ip address 10.10.10.1 255.255.255.248
ip http server
no ip http secure-server
ip dhcp pool SDM
network 10.10.10.0 255.255.255.248